1. Field of the Invention
This invention relates to integrated circuits of the type known as microprocessors and more particularly to microprocessors containing as a part thereof a programmable read-only memory (ROM) suitable for storage of a program controlling the operation of the microprocessor.
2. Description of the Prior Art
Microprocessors are now well-known in the semiconductor industry. A typical microprocessing system is described in U.S. Pat. No. 3,984,813, issued Oct. 5, 1976 on an application of David H. Chung. The microprocessor shown and described in that patent comprises a minimum twochip system wherein one chip includes the central processing unit (CPU), a scratch pad RAM memory, an interrupt circuit and several input-output (I/O) ports. A second chip contains a program counter, a ROM circuit, an interrupt and additional I/O ports. In operation, the ROM circuit contains the program controlling the operation of the associated central processing unit. Information transmitted to the microprocessor is then processed in accordance with this program and output signals generated by the operation of the CPU are sent from the microprocessor to carry out selected tasks. The particular program contained in the ROM circuit is usually proprietary to the party using the microprocessor. However, because of the way in which microprocessors are manufactured, the ROM program must be placed in the ROM by the manufacturer of the microprocessor and not by the customer who develops the program. Accordingly, the customer is concerned that the program be held by the manufacturer in confidence and not be disclosed either directly or indirectly to third parties.
The manufacturer, on the other hand, is faced with a difficult logistics problem. Typically, a given type of microprocessor will be sold to many thousands of different customers, each with a different proprietary program to be manufactured into the ROM. The manufacturer must be able to determine which particular microprocessor chips have been programmed for specific customers and must develop procedures to ensure that a given customer receives only those microprocessor ROM chips containing that customer's program. Furthermore, each customer is also concerned that his own customers or the ultimate users of the microprocessor will not have access to the customer's proprietary program placed in the ROM chip by the manufacturer. By allowing the ultimate user of the microprocessor to have access to this code, the user could then seek bids from other manufacturers of microprocessors in an attempt to obtain the microprocessors from other sources.
Accordingly, a method is required to enable a manufacturer of microprocessors to specifically identify the particular ROM chips destined for particular customers in a unique and fail-safe manner and to allow the customer to prevent his customers, the ultimate users, from reading out the particular proprietary program generated for use in the microprocessor.